Chemical ionization is generally a 3-stage process. In the first stage primary ions of the reactant gas are generated by electron impact. In the second stage secondary ions of the reactant gas are generated by ion-molecule reactions of the primary ions with the molecules of the reactant gas. In the third stage the required ions of the analyte gas are created by ion-molecule reactions with the secondary ions of the reactant gas. In this process there are very many side-reactions and branches of the reaction path which lead to undesirable reaction products.
Ion-molecule reactions have been studied for a long time to clarify reaction mechanism. It has been particularly the reaction cross-sections which are of interest, and the dependency of the reactions on gas temperature, concentration of the reaction participants, the impact energies, and other parameters.
Study of the ion-molecule reactions has been performed in ion beams, in plasma, in space-charge-bound ion clouds (locked into an electron beam), but particularly also in both types of ion trap mass spectrometer: ion cyclotron resonance spectrometers (ICR) and RF quadrupole ion traps. The reactions are examined by mass-spectrometric analysis of the resulting product ions.
There is particular interest in ion-molecule reactions as probes for certain structural properties of the neutral molecules examined. For this purpose it is necessary to have output ions of high purity for the reactions in order to prevent contamination of the product ion spectrum examined by undesirable by-products.
Elimination of undesirable ions has been well known for a long time now with regard to ICR spectrometers. One has also been able to isolate or store selectively ions of uniform mass-to-charge ratios in quadrupole ion traps.
For the study and application of ion-molecule reactions, however, it is frequently necessary to select specific output ions for a certain reaction using a reaction path which involves predecessor ions and it is therefore desirable to suppress side-reactions. So far there have been special methods available for this.
Chemical Ionization (CI) is a widespread method for ionizing substances. When used properly it provides information about the molecular weight of a substance. Spectra of mixtures can be interpreted more easily because far fewer fragmented ions are created than with electron impact spectra. Measurement of the molecular ions is also important for clarification of the structure of the substances. Chemical ionization is advantageous particularly for applying MS/MS methods to investigating molecular ions. There are other applications in fast quantitative analyses of substances in mixtures.
As various reactant gases and various ionization reactions are available for chemical ionization, the method can be very well adapted to the aim of the investigation. The method is described in all the relevant, more recent mass spectrometry textbooks, for example, in Odham, Larsson and Mardh's "Gas Chromatography/Mass Spectrometry, Applications in Microbiology", Plenum Press, New York and London, 1984.
Chemical ionization is described as "soft" ionization which ionizes more carefully than the relatively "hard" electron impact ionization. Careful ionization means that during the ionization process there is virtually no fragmentation of the molecular ion because only small further quantities of energy are transferred to the molecule in addition to the ionization energy. The usual electron impact ionization with 70 to 100 eV electrons shows no measurable molecular ions for approx. 30% of all substances but only fragmented ions; unambiguous identification of the substances is thus hindered. Almost with regularity, chemical ionization, on the other hand, shows a "pseudomolecular ion" which has been obtained by protonation and the mass of which is just one atomic mass unit larger than that of the molecular ion. By suitably selecting the reactant gas and the ionizing reaction it is possible to suppress almost completely any ensuing fragmentation of the pseudomolecular ion.
Chemical ionization (CI) usually takes place in three ionization stages from a mixture of gases. The gas mixture consists of a largely inert carrier gas which plays no role in the reactions (also called collision gas), a reactant gas for forming the proper reactant ions for chemical ionization, and the analyte or test gas which is to be ionized chemically. The latter can also be a mixture of various substance gases or vapours if at the same time more than one substance is to be subjected to a qualitative or quantitative investigation.
In standard ion sources for chemical ionization, which are operated at a pressure of about 1 millibar, the reactant gas has a much higher concentration than the analyte gas. Therefore, in an initial stage of electron impact ionization (EI) it is predominantly primary ions of the reactant gas which are formed. The EI ions of the analysis gas which are created at the same time are relatively small in number but they subsequently form background noise in the final CI spectrum of the analysis gas ions.
In ion traps which are operated at pressures of 10.sup.-4 to 10.sup.-3 millibar such a high ratio of concentrations of reaction gas and analysis gas cannot be set because if it were, the reaction times would be too long due to the pressures being much lower. For this reason, in ion traps there are far more fragmented ions resulting from EI ionization if no further protective measures are taken.
The primary ions of the reactant gas which are formed in the phase of electron impact ionization are naturally not uniform but, in turn, consist of the molecular and fragmented ions of the reactant gas, in which one type of ion (ions of a single mass) usually largely predominates. For convenience, this predominant type of primary ion is referred to herein as the "main primary ion". The main primary ions are generally not suitable for subsequent chemical ionization for energy and structural reasons. The main primary ions of the reactant gas, however, then react in a second stage with molecules of the reactant gas in ion-molecule reactions, "main secondary ions" of the reactant gas being formed. The main secondary ions have a structure and an energy balance which allow them to react with the molecules of the analyte gas in a third stage, forming the CI analyte gas ions.
The structure necessary for chemical ionization is generally characterized in that a proton is relatively loosely bound to a residual molecule which otherwise has a very stabile energy balance. Chemical ionization is largely a protonation reaction. In rarer cases it is a methylation reaction or the transfer of an even larger charged fragment. In general the aim is to achieve undisturbed protonation.
Example: during chemical ionization with water as reactant gas the primary ions H.sub.2 O.sup.+ ' are initially formed. Also created are the subprimary ions OH.sup.+, of which there is a large number, but they have no further involvement here. The primary ions H.sub.2 O.sup.+ ' react with other water molecules according to the equation EQU H.sub.2 O+H.sub.2 O.sup.+ '=OH'+OH.sub.3.sup.+ ( 1)
forming the free radical OH' and the main secondary ion OH.sub.3.sup.+. The main secondary ion OH.sub.3.sup.+ then reacts with molecule M of the test substance in accordance with the equation EQU OH.sub.3.sup.+ +M=H.sub.2 O+MH.sup.+ ( 2)
forming the "pseudomolecular ion" MH.sup.+. This protonation reaction (2) is the "chemical ionization" proper.
Naturally with all these processes there are a number of side reactions which may lead to other CI analysis gas ions. The side reactions can be caused by subprimary ions but they are chiefly generated by side reactions of the main primary ions which then lead to subsecondary ions. The subsecondary ions can then, with their own CI reactions, lead to slightly different CI analysis gas ions.
Apart from the protonating CI reaction, which is really desired, there are other ionization reactions of the main secondary ion taking place for this reason, which, under certain circumstances, may lead to large quantities of fragmented ions of the test molecule. It depends on the aim of the investigation as to whether these are desirable or undesirable. For energy reasons the fragmented ions are generally all the more frequent, the smaller the ionizing main secondary ions are.
Sometimes there are even two competing CI reactions of the same reactant gas, whereby the formation of fragmented ions is different for the two reaction chains. Example: The frequently used chemical ionization with methane as reactant gas initially leads to the EI ions CH.sub.3.sup.+ and CH.sub.4.sup.+ ', the relative frequency of which depends on the energy of the ionizing electrons. These electrons react with other methane molecules forming two different main secondary ions. EQU CH.sub.3.sup.+ +CH.sub.4 =H.sub.2 +C.sub.2 H.sub.5.sup.+ ( 3) EQU CH.sub.4.sup.+ +CH.sub.4 =CH.sub.3 '+CH.sub.5.sup.+ ( 4)
Under normal conditions at a pressure of 0.7 millibar about 48% ions are type CH.sub.5.sup.+ and 40% type C.sub.2 H.sub.5.sup.+, the remainder being accounted for by even heavier ions. The main reactions for the subsequent chemical ionization are: EQU C.sub.2 H.sub.5.sup.+ +M=C.sub.4 H.sub.4 +MH.sup.+ ( 5) EQU CH.sub.5.sup.+ +M=CH.sub.4 +MH.sup.+ ( 6)
whereby the second reaction (6) provides more subsequent fragmentations than the first one.
As is evident from this, chemical ionization can be adapted to the problem of the investigation. Consequently, chemical ionization is generally always "softer" whenever one proceeds to heavier reactant gas ions or selects the correspondingly heavier main secondary ions. If chemical ionization with water (OH.sub.3.sup.+ as main secondary ion) is still relatively hard, it is increasingly softer in the sequence ammonia (NH.sub.4.sup.+), methane (CH.sub.5.sup.+, C.sub.2 H.sub.5.sup.+), isobutane (C.sub.4 H.sub.9.sup.+ and many others).
For chemical ionization in an ion trap a mixture is usually admitted made up of a collision gas of low molecular weight, a reactant gas for forming the output ions for chemical ionization, and an analyte gas to be tested. In ion traps, however, no such high concentration ratio can be set as is permitted by the ion sources usually operated at a pressure of 1 millibar for other types of mass spectrometer. For this reason, in ion traps it is particularly disadvantageous that the fragmented ions of the test substance cannot be removed from electron impact ionization and remain visible in the CI spectrum of the substance.
To eliminate this disadvantage, EP 0 215 615 B1 describes a method whereby initially only light ions in a certain mass range are stored. The range of light ions covers both the primary and secondary reactant gas ions as well as the light fragmented ions which result from the analysis gas molecules by electron impact. In a second stage in which the storage conditions of the ion trap are changed, storage of the CI ions of the analysis substance are then admitted. The CI ions are then identified by the special method of ion ejection by mass-sequential stability at the limit of the stability range in the a,q diagram, including the remaining electron impact ions.
Consequently, the disadvantage of chemical ionization in ion traps compared with standard CI ion sources is partially offset. There is still the deficiency that the remainders of electron impact ionization are not eliminated and also that there is no clear selection of reaction paths.
Due to the publication "A New Mode of Operation for the Three-Dimensional Quadrupole Ion Store (Quistor): The Selective Ion Reactor" by J. E. Fulford and R. E. March (International Journal of Mass Spectrometry and Ion Physics 26 (1978) 155) a method is known where during electron impact ionization the main primary ions in the ion trap are freed of all other ions because the working point of the main primary ions with a pulse of DC voltage is briefly shifted into the corner of the stability diagram. Consequently all the other ions are transferred to instable areas and thus removed from the ion trap. After the end of the pulse the reactions required can then take place with the analysis gas molecules. The resulting product ions are then expelled from the ion trap by a pulse. They pass through a quadrupole filter with which in any experiment one type of ion can be identified. By cyclic repetition of the experiment with different filter values for the quadrupole filter the entire spectrum can be scanned.
The disadvantage of this method is that only the main primary ions of single-stage CI processes can be isolated. The single-stage CI processes in this work are generally based on the so-called charge exchange (CE) reactions but these are much less interesting and are not even classified as CI processes by some authors. Moreover, this method can only provide relatively few of the desired reactant gas ions for physical reasons (depth of potential wall). Even in the event of a combination with one of the modern standard methods of scanning which identify all the ions in the ion trap by mass-sequential ejection, there is still the disadvantage of not being able to eliminate side reactions of the main primary ions.
The present invention seeks to provide a method which permits tailored chemical ionization of the molecules of the test substance by secondary reactant gas ions without the formed CI ions of the test substance being contaminated with fragmented ions from electron impact ionization or with product ions from side reactions. In the same way it should be possible to provide suitable output ions for ion-molecule reactions.
From U.S. Pat. No. 4,761,545, EP 0 362 432 A1, and U.S. Pat. No. 5,134,286 it is known that an ion trap can be subjected to a frequency mixture in such a way that storage of ions with selected mass-to-charge ratios is prevented. The frequency mixture is applied as a mixture of voltages with various frequencies to the electrodes of the ion trap and inside the ion trap generates corresponding alternating fields which can cause the secular oscillations of the ions in the ion trap to perform resonant energy absorption dependent on mass.
If the frequency mixture is applied to the two end caps of the ion trap in phase opposition, a dipolar alternating field is created in the ion trap with excitation of the oscillations in the axial direction of the ion trap. If the frequency mixture is applied to the two end caps in phase, hence phase opposition between the ring electrode on the one hand and the end cap electrodes on the other, a quadrupolar alternating field is generated in the ion trap which can excite the secular oscillation both in the axial and in the radial direction.
In particular it is possible to generate the frequency mixture in such a way that due to integrated frequency gaps only a few types of ion with selected mass-to-charge ratios can be stored. Furthermore it is possible to adapt the frequency mixture during a phase of electron impact ionization so that only certain ions of the resulting ion mixture are stored.
Therefore, it is an object of the invention to select product ions of the desired two first stages of the reaction path in such a way that they are stored on their own and to suppress the product ions of undesirable reaction paths.